1. Field of the Invention
The present invention is directed to a method in the form of a pulse sequence, for operating a magnetic resonance imaging (nuclear magnetic resonance tomography) apparatus.
2. Description of the Prior Art
The T.sub.2.sup.* contrast is of particular significance for functional imaging in a magnetic resonance tomography apparatus. The T.sub.2.sup.* contrast is the effective transverse relaxation time, into which the signal decay due to spin-spin couplings and the influence of local field inhomogeneities. Effects that influence the T.sub.2.sup.* relaxation time are usually exploited for the functional imaging. These can be intrinsic effects as occur, for example, in the oxygenation/de-oxygenation of blood. Magnetic contrast agents that are distinguished from tissue by an especially pronounced T.sub.2.sup.* contrast, however, can also be employed.
The T.sub.2.sup.* contrast is directly dependent on the echo time, i.e. on the interval between the excitation of a nuclear magnetic resonance signal and the read-out of the echo signal. Long echo times must therefore be selected to obtain a high T.sub.2.sup.* contrast. This leads to long data acquisition times. Short data acquisition times are required, however, to achieve functional imaging of rapidly occurring processes.
In order to solve this problem, Lin G. et al, SMRM Abstract 11, page 1817 (1992), proposed a method that is referred to therein as "echo-shifted FLASH". The echo signal allocated to an excitation pulse is thereby acquired only in one of the subsequent repetition periods, i.e. between two following excitations. The desired, long echo times are thus obtained even given short repetition times, and thus short measuring times. The same slice is always excited in this method. In order to dephase undesired echoes, additional gradient pulses are provided. The dephasing of undesired echoes, however, is not completely successful in practice, so that the method is relatively susceptible to artifacts.
Bishop et al in SMRM-Abstracts 9, page 437 (1990), disclose a multi-slice technique wherein a plurality of slices are successively excited within an echo time. Spin echoes are generated by locking 180.degree. radio-frequency pulses onto the same slices. With this method, a substantial time gain can be achieved when measuring a plurality of slices. Due to the use of spin echoes, however, local magnetic field inhomogeneities are largely compensated, so that the resulting images exhibit no T.sub.2.sup.* contrast, but, instead exhibit a T.sub.2 contrast. This method is therefore not suitable for functional imaging wherein effects to which only the T.sub.2.sup.* relaxation time is susceptible play a critical role.